A typical discharge lamp for such medical and cosmetic applications comprises a xenon arc flashlamp located within a reflector shaped to direct the optical output from the flashlamp to a treatment site.
The flashlamp is typically driven by a capacitor discharge circuit where the electrical energy required is stored in a capacitor until the output optical pulse is required. When the optical output is required, the electrical energy is delivered to the flashlamp, thereby converting the electrical energy to optical output.
In such an arrangement, the current flowing through the flashlamp varies during the pulse, proportional to the discharge characteristics of the capacitor. This variation in the current during the pulse produces a varying intensity of optical energy and induces a shift in the output wavelength spectra as the output wavelength is determined by the plasma temperature within the flashlamp, and the plasma temperature is governed by the current flowing.
Referring to FIG. 1A of the drawings, there is illustrated a simplified version of a conventional flashlamp drive circuit, in which a power supply unit 100 is used to charge a relatively small capacitor 102, in this case say 500 μF. A switch 104 is provided between the capacitor 102 and the flashlamp 106. Examples of switches used in the past have included thyristors, which once turned on, generally remain on until the capacitor has fully discharged, and transistors. When the switch 104 is closed, the capacitor 102 is substantially completely discharged to the flashlamp 106, giving a drive current pulse similar to that illustrated in FIG. 1B, whereby around (say) 150 J of energy (defined by the area under the curve in FIG. 1B) is delivered to the flashlamp in around 5 ms.
However, there are applications, particularly medical applications, where the shape of the optical pulses used to drive the flashlamp is important in order to achieve the desired therapeutic effect, and in particular to achieve such effect without damage to areas of the patient's body not being treated. For example, in optical dermatology, it may be desirable to rapidly heat a target chromophore to a selected temperature, and to then reduce applied energy so as to maintain the chromophore at the desired temperature. It is therefore highly desirable for the shape and duration of the optical pulses delivered to the flashlamp to be controllable.
Referring to FIG. 2A of the drawings; there is illustrated a simplified form of another known flashlamp drive circuit, in which a power supply unit 100 is used to charge a relatively large capacitor 102 (say, 0.2 F) up to, say 1500 J, and a switch 104 (embodied in this case by a transistor) is used to deliver a small portion of this total energy (say 150 J) at a time. In view of the manner of operation of this type of partial discharge system, an optical pulse can be delivered to the flashlamp 106 with a relatively uniform energy distribution, as illustrated in FIG. 2B of the drawings. Effectively, a drive system of the type illustrated in FIG. 2A of the drawings, delivers a plurality of small packets 108 of energy. Thus, in the case where 150 J of energy are delivered in a 50 ms-time interval, each packet 108 will consist of 0.03 Jμs. As a result, it is possible, using such a system, to control the shape of the optical pulse delivered to the flashlamp in order to achieve the desired effect.
However, a major disadvantage of the partial discharge system described with reference to FIG. 2A of the drawings, is the size of the capacitor 102, whereas it is highly desirable in all flashlamp applications to minimize the size of the capacitor (and therefore the charge it carries) as this has the effect of minimizing the size, weight and cost of the lamp drive circuitry and enhances the safety of such drive circuits by reducing shock risks.
A method aimed at producing a constant current during the optical pulse is proposed in U.S. Pat. No. 6,888,319. This approach provides a drive circuit for a pulsed flashlamp which circuit includes a sensor for power through the lamp, and a series regulator which operates an on/off switch between the energy storage capacitor and the flashlamp, the switching frequency being determined by monitoring the current flow or power within the circuit. This approach can provide a relatively constant current output during the overall current pulse and is commonly referred to as a flywheel circuit as described in, for example, U.S. Pat. No. 4,513,360.
Whilst providing a constant current pulse does have advantages, this approach does not provide constant optical output, because the output optical power can depend upon many external factors that are not manifested as variation in current. These factors include, but are not limited to, gas fill pressure, gas purity, operating temperature, flashlamp envelope degradation, flashlamp envelope coating (often flashlamps are coated to improve conduction) or flashlamp envelope doping (doping the envelope can selectively filter certain wavelengths). Many of these parameters can vary during usage; for example, it is common for flashlamp output to degrade through usage as contaminants can cause optical fluctuations. Such contaminants can cause optical degradation but may not affect the current flowing in the flashlamp.